Understanding the mechanisms associated with chemical catalysis is vital for not only the rational improvement of their capabilities, but also for the advancement of the fundamental knowledge affiliated with the systems in question. These new insights can help predict new reactivities towards more challenging substrates which will allow for easier access to complex organic targets. More specifically, accessing the -position of pyridines and the carbonyl bonds in aldehydes and ketones are important as they are prominent structural motifs in many pharmaceuticals. A lutetium based catalyst [Cp*2LuCH(TMS)2 (Cp* = 5-C5Me5) (TMS = SiMe3)] is shown to be a competent catalyst for the C-H -mono-borylation of a variety of pyridines. An in-depth mechanistic interrogation of the system, along with computational details, will reveal a plausible mechanism involving an 2-lanthanide-azine complex. Interestingly, by simply changing the lanthanide and keeping all other components constant, the reactivity diverges: smaller and more electrophilic lanthanide ions favor -C-H functionalization while larger and less electrophilic lanthanide ions favor a B-N bond-forming 1,2-dearomatization of pyridines. It will be seen that these two reaction pathways share an intermediate, which is presumably the point where the selectivity diverges. The Marks’ lab recently reported a rapid and selective hydroboration of aldehydes and ketones using a homoleptic lanthanide trisamido catalyst, however, the mechanism is still unknown. Experimental and theoretical mechanistic data presented here supports the intermediacy of a coordinated carbonyl oxygen to the lanthanide center, followed by a ligand-assisted hydroboration of the carbonyl bond with pinacolborance (HBpin). Utilizing NMR and X-ray crystallography, key intermediates were studied alongside stoichiometric test reactions. One structure is a ketone coordinated to the lanthanum center via the carbonyl oxygen and is active in the hydroboration of ketones. In addition, two off-cycle intermediates are obtained: one was observed when the La catalyst reacts with excess HBpin, the other reveals an insertion of one of the amido ligands into the substrate. Finally, DFT calculations are presented supporting a ligand-assisted mechanism which is in good agreement with experiment. These new mechanistic insights help explain previous research findings and reveals a unique ligand assisted hydroboration mechanism.

Creator

• Rothbaum , Jacob Olasov

DOI

• https://doi.org/10.21985/n2-fd5r-4317

Subject

• Chemistry

Language

• en

Alternate Identifier

• http://dissertations.umi.com/northwestern:16303
• etdadmin_upload_936040

Keyword

• Catalysis
• Lanthanides
• Borylation
• Chemodivergent

Date created

• 2022-01-01

Resource type

• Dissertation

Rights statement

• In Copyright